Military and commercial users seek high-precision micro-gyroscopes (“micro-gyros” herein) fabricated from MEMS processes for a number of applications including use in guidance systems and for sensing angular rotation in vehicles and equipment.
For the next generation of high-precision micro-gyro, a heightened set of performance specifications must be met; particularly for military and space applications. Current technology trends suggest the need for a micro-gyro having a bias stability better than <0.01 deg/hr, an angle random walk of better than <0.003 deg/rtHr and an azimuth accuracy of at least ±1 mil.
Prior art vibratory micro-gyros are ideally suited for hand-held applications due to their small size, low weight and low power. Vibratory micro-gyros are widely used in the aerospace, consumer and automotive markets. Prior art vibratory micro-gyros are typically micro-machined using established MEMS foundry processes, are fabricated from silicon or quartz materials and can be produced at relatively low cost (less than $10/unit) in automotive quantities.
Unfortunately, existing MEMS vibratory micro-gyros cannot meet the challenging performance specifications set forth above.
Relatedly, despite a decade of development and hundreds of million dollars of investment, prior art vibratory micro-gyros have also not yet met the performance specifications of the more accurate, higher tolerance prior art optical micro-gyros.
Prior art optical gyros, on the other hand, have not been able to reduce their size and manufacturing costs, thus limiting their application to expensive systems such as in aircrafts and missiles.
One major factor that limits the performance of the above prior art micro-gyro devices is the signal processing methodology used to process the output signal of the micro-gyro itself. The methods used are based primarily on the physical operation and structure of such devices.
Almost all micro-gyros function in part as amplitude modulated (AM) transducers. The instant invention discloses a new type of micro-gyro signal processing method that processes the frequency-modulated (FM) output signal of a frequency-modulated micro-gyro, i.e., a micro-gyro that produces an output signal with a frequency that is modulated by the input rate. The method of the invention dramatically improves noise suppression by about two orders of magnitude as compared to prior art micro-gyros operating based on amplitude modulation.
Due to the small physical structure of prior art micro-gyros, the proportionately small output signals require large amplification. Accordingly, it is well-understood by those skilled in the art of MEMS micro-gyro technology that a major limiting factor in micro-gyro performance is noise in the associated signal processing electronics. A micro-gyro element that generates an FM output signal on the other hand beneficially provides an efficient output signal to minimize the noise element in a micro-gyro output signal.
Before detailing the signal processing methods that are the subject of the invention, the basic elements of an exemplar prior art FM micro-gyro structure are briefly discussed. Such a prior art FM micro-gyro is disclosed for instance in the United States utility patent application entitled “Frequency Modulated Micro Gyro”, U.S. Pub. No. 2010/0095770, dated Apr. 22, 2010 to Hsu, the entirety of which is incorporated herein by reference.
Turning now to the figures wherein like numerals define like elements among the several views, FIG. 1 depicts the fundamental structural elements of a prior art FM micro-gyro.
In the depicted embodiment of FIG. 1, the FM micro-gyro structure comprises a single monolithic drive element fabricated from a MEMS process wherein the drive element is suspended in space, making contact with the substrate at the central anchor. The exemplar FM micro-gyro structure comprises a drive element (also referred to as a “proof mass” herein), a central anchor, a drive electrode, an oscillator, an oscillator anchor and a support flexure.
The structure defining the oscillator sense elements and oscillator anchor comprises a resonator structure.
The FM micro-gyro is disposed in space with respect to a drive axis, a rate axis and a sense axis.
The underlying principal behind an FM micro-gyro is similar to the phenomenon observed in the tuning of a guitar string. This example can be used to illustrate the principal of converting the Coriolis force into a shift in the resonant frequency of an oscillator sense element on a resonator. The oscillator sense element's resonant frequency is determined by its mass and its stiffness. The Coriolis force is applied to one end of the oscillator sense element with the other end connected to the stationary oscillator anchor.
The change in the tension of the resonator's oscillator sense elements causes a shift in the resonant frequency of the oscillation, in the same manner that the increasing or decreasing of the tension on a guitar string changes the resonant frequency of the string.
In operation, the drive element on the FM micro-gyro is electronically driven to oscillate rotationally about the drive axis such as by an electrostatic drive means. When the oscillating element experiences an angular rate change (also referred to as an input rate) about the rate axis, a Coriolis force is generated about the sense axis. Directing the resultant Coriolis force to vary the tension on the oscillator sense element provides a means to shift the resonant frequency of the one or more oscillator sense elements.
Capacitive electrodes are used to actuate and detect all elements in the device. The sense oscillator is configured such that the output Coriolis force creates a shift in the resonant frequency of the oscillator sense element. A sensed change in capacitance is implemented in the device to convert movements of the oscillator sense elements into a voltage or current output signal. The output signal is then passed through a signal processing FM detection circuit to extract the rate change information.
Prior art vibratory micro-gyros (as distinguished from FM micro-gyros) generally operate and produce an output signal based on an input rate in the following manner: A drive element is driven to oscillate at a predetermined resonant frequency about the drive axis of the device. When the oscillating element is subjected to an angular velocity about the rate axis, a force is generated about the sense axis; all three axes being orthogonal to one another. The resulting Coriolis force has a magnitude that is proportional to the product of the oscillator's mass, its velocity and its angular rate. Virtually all vibratory micro-gyros rely on this same Coriolis principle of operation for sensing the angular rate of the device.
In the instance of vibratory micro-gyros, the Coriolis force is extremely small (pico-Newtons) and is calculated by measuring the micro-rotation of the drive element about the sense axis. At very low angular rates, the movement of the element may only be about the size of an atom. This very small movement is typically detected by changes in capacitance between two elements in the device, which capacitance change is measured using a suitable readout circuit.
Ultimately, the output of the readout circuit from a vibratory micro-gyro provides an electronic signal that is proportional to the amplitude of an input angular rate change.
As can be seen, vibratory micro-gyros effectively have an output signal that is amplitude modulated (AM) by an angular rate.
In contrast, the frequency-modulated micro-gyro such as depicted in FIG. 1 produces an output signal that is frequency-modulated by the angular input rate. The FM micro-gyro has at least one oscillator sense element connected by an oscillator anchor support beam to the drive element where a responsive Coriolis force is directed to alter the tension thereon and shift the resonant frequency of the oscillator sense elements.
As contemplated by FIG. 1, an output Coriolis force will cause the ring-shaped drive element to rotate about the “sense axis”, thus altering the tension on the oscillator anchors connected to the oscillator sense elements. Electrodes underneath the drive element are connected to actuate and sense the elements by electrostatic effects.
The above-referenced FM micro-gyro technology provides important advantages when compared to micro-gyros operating based on amplitude modulation. These advantages include:
1. FM Gyros have high resolution and low noise when compared to AM micro-gyros: A high-performance AM micro-gyro is generally limited to measuring amplitude changes of 0.1 deg/sec over 100 deg/sec in full scale, or 0.1%. Undesirably, numerous noise sources exist in AM micro-gyros including amplifier noise, voltage reference noise, and resistor noise. Despite the best filtering and demodulation techniques, a significant amount of noises still passes through in these devices, limiting the resolution.
2. Frequency stability: The stability of micro-gyros is due in large part to stability of its signal processing electronics. The slow shift in voltage sources is common and is an example of a component or system issue that limits the performance of a prior art micro-gyro having, as an example, a typical voltage stability of about 50 ppm.
On the other hand, an FM micro-gyro relies on a frequency source for the control of its oscillator sense elements. Very stable frequency sources are readily available with stability of just a few ppm. A stable reference source combined with excellent filtering in FM signal processing technology leads to superior performance in an FM micro-gyro as compared to an AM micro-gyro.
3. High bias stability: A bias stability specification of <0.01 deg/hr is about two orders of magnitude higher than commercially produced prior art “high performance” micro-gyros. Despite tremendous advances made in the performance of current micro-gyros, an order of magnitude improvement is not expected and unlikely as is available from an FM micro-gyro.
4. Low angle random walk: Angle random walk is a direct measure of noise in a micro-gyro. The superior noise filtering techniques available in FM signal processing electronics enable an FM micro-gyro to far exceed the performance of AM micro-gyros. The use of FM detection circuitry provides up to a two order of magnitude in noise reduction over prior art devices.
Because the shifts in the output frequency of an FM micro-gyro are not large, the invention herein is directed at providing signal processing approaches for measuring ultra-low frequency shifts resulting from an input angular rate change of an FM micro-gyro.
In a first aspect of the invention, a first method for processing an output signal of an FM micro-gyro uses a cumulative time difference technique referred to herein as a windows subtraction method. The windows subtraction method can detect a frequency shift as small as 0.1 Hz on, for instance, an exemplar 2 MHz resonator and enables an FM micro-gyro to measure very low rotational rates in the range of 1e-5 deg/sec.
The windows subtraction method desirably has a low sensitivity to jitter noise and is implemented using relatively simple hardware. The time for signal processing for this method is as low as between about 0.3 and 2.5 seconds, depending on the level of confidence desired by the user or application.